Internal combustion engines produce exhaust gases that include undesirable combustion byproducts, such as oxides of nitrogen (NOX). Emission control systems may be employed to reduce NOX emissions. For example, NOX reduction catalysts, including selective catalytic reduction (SCR) catalysts, are utilized to convert NOX (NO and NO2 in some fraction) to N2 and other compounds, such as water (H2O). SCR catalysts utilize a reductant, such as ammonia (NH3), to reduce the NOX. Emission systems may also utilize an alternate compound, such as urea, which vaporizes and decomposes to ammonia in the exhaust gas stream. These compounds and the exhaust gases react with a catalyst to help reduce emissions. Currently available SCR catalysts can produce high NOX conversion rates.
SCR catalysts generate ammonia to reduce the NOX. When just the right amount of ammonia is available at the SCR catalyst under the proper conditions, substantially the entire quantity of ammonia is utilized to reduce NOX, with only a small portion, if any, exiting the SCR catalyst. If the reduction reaction rate is too slow, however, or there is excess ammonia in the exhaust gas stream, ammonia can slip out the exhaust pipe. Further, when using urea under certain operating conditions, the urea may decompose to ammonia downstream of the SCR, causing ammonia slip and less ammonia available for NOX reduction and higher NOX emissions at the tailpipe. To help minimize ammonia slip, SCR systems may utilize an ammonia oxidation (AMOX) catalyst arranged downstream of the SCR catalyst to reduce at least a portion of ammonia slipping from the SCR catalyst to N2 and other less harmful compounds.